Finding angular velocity of telescopic arm

So ,here is the question : The structure is shown in the picture.
All I need is to find the angular velocity of telescopic arm BC ,in this current position.

http://img197.imageshack.us/img197/3954/w6e.JPG [Broken]

2. Relevant equations

Well,them simple ω=rXv/r[itex]\bullet[/itex]r

3. The attempt at a solution

Here is my attempt,and I don't know why it is wrong .

I said that ω[itex]_{bc}[/itex]=ω[itex]_{b}[/itex]-ω[itex]_{c}[/itex]

Then I calculated ω[itex]_{b}[/itex]=rXv/r[itex]\bullet[/itex]r , same with ω[itex]_{c}[/itex] .
The velocity of those points were simply r*ω of the joints.
I set the coordinates O point in the lower left corner . Anyway,I got the correct angular velocities next to that system of coordinates.
But when I tried to say ω[itex]_{bc}[/itex]=ω[itex]_{b}[/itex]-ω[itex]_{c}[/itex] , I got the wrong answer .
Why is that ?
What is the correct way to look at that problem ?